NEVE DEKALIM, Gaza Strip, Aug. 15 -- Thousands of Israeli soldiers entered Jewish settlements in the Gaza Strip on Monday to deliver eviction notices to be carried out, with force if necessary, in the coming days. But a well-organized opposition prevented soldiers from entering some settlements and ended the day confident that it was still possible to block the Israeli withdrawal.

In a nationally televised address Monday evening, Prime Minister Ariel Sharon said violence against Israeli settlers and soldiers in Gaza and the threat posed to Israel's Jewish majority by the fast-growing Palestinian population had prompted his decision to abandon communities he once championed.

"It is no secret that I, like many others, believed and hoped that we could forever hold on to Netzarim and Kfar Darom," said Sharon, referring to two of Gaza's 21 settlements. "However, the changing reality in this country, in this region, and in the world, required another reassessment and changing of positions."

The demonstrations in half a dozen settlements marked a slow, frustrating start to an evacuation plan Israeli officials hope to conclude within three weeks. Although government and military officials attached little significance to the resistance, the day pointed to a possible confrontation when the grace period for the settlers to leave on their own expires at midnight Tuesday.

The army managed to serve 75 individual eviction notices in three settlements in the north and handed the leaders of several others blanket orders for all homes to be vacated. Soldiers did not visit several notoriously defiant communities, where the young shock troops of the religious settlers' movement from the West Bank have encamped in recent weeks.

"This was a difficult day for all of us -- the evacuees, the evacuators and the public of Israel," said Eyval Giladi, Sharon's director of strategic planning. "We will be more decisive with the young infiltrators" in Gaza.

Army officials said the level of resistance was expected given the anger generated by Sharon's plan to withdraw from Gaza -- where 8,500 Israelis live among 1.3 million Palestinians -- after nearly four decades. The disengagement plan also calls for the evacuation of four smaller West Bank settlements once the Gaza withdrawal is complete.

A vocal minority believes that leaving Gaza without Palestinian concessions runs counter to Israel's security interests, while religious nationalists oppose leaving any land described in the Bible as part of ancient Israel. But there were signs Monday that many Gaza settlers were leaving on their own. Israeli officials said 31 moving containers arrived here in Neve Dekalim, the largest Gaza settlement, while residents of the northern community of Nissanit removed Torah scrolls from their synagogue and began dismantling it.

In his address, Sharon said, "It is out of strength and not weakness that we are taking this step. We tried to reach agreements with the Palestinians which would move the two peoples towards the path of peace. These were crushed against a wall of hatred and fanaticism. The unilateral disengagement plan, which I announced approximately two years ago, is the Israeli answer to this reality."

In the next phase of the withdrawal, the military will begin forcibly removing what will likely be several thousand Israelis still in the settlements, a prospect that disturbed some soldiers after a bitter day intended to be a final offer of help. After hours of scuffles, shouting and occasional fistfights, several hundred soldiers managed to enter only the edge of Neve Dekalim before officers decided against going in to serve eviction notices.

Young men in the knit skullcaps that are the trademark of religious settlers slashed the tires of army jeeps, sprayed dishwashing liquid on windshields to limit visibility and ripped out the radio of one military vehicle. Others locked arms in human chains, sang songs of love for the soldiers and delivered angry lectures to the mostly implacable young officers overseeing the operation.

Silhouetted in the middle distance, scores of Palestinians gathered along a ridge in the city of Khan Younis, watching the demonstration ebb and flow on what was reportedly the hottest day of the year. Rifle fire clattered regularly from an Israeli military watchtower nearby, and two shots that Israeli military officials said were fired by a Palestinian sniper cracked on the pavement among the soldiers and settlers.

For the 2nd Company of the Shachar Battalion, the day that its soldiers had spent months training for started before dawn in the fog at a military base just outside the Gaza Strip. Only a few months ago, the army camp was a field of sunflowers.

The battalion's 450 troops were handpicked from the ranks of professional soldiers for the disengagement operation. But despite their experience, a nervous quiet marked the hours before they filed onto buses -- many of them unarmed, given the spirit of the day's operation -- for the trip to the settlements.

"You can't just have an 18-year-old soldier pulling people out of their houses," said Maj. Max Levy, 27, the battalion's operations officer. "These are people who have homes and know what that means."

Along the road, progress was halting, and the bus idled frequently as word passed over army radios that hundreds of demonstrators were blocking the way ahead. Some soldiers fidgeted with cell phones, others recited from pocket-size tehilim -- the book of psalms -- while they waited for the buses to move.

"Should I be doing this? I don't know, I really don't know," said Rami Marom, 30, a noncommissioned officer who lives in the West Bank settlement of Shaked and disagrees with the evacuation policy. People in his settlement tell him that he "shouldn't be here," he said, but "I tell them I am in the military, that I have to do my job."

During the pauses, Maj. Yitzhak Nachmani, the company commander, strolled up the aisles. Compact and ebullient, Nachmani patted each soldier on the shoulder, seeking to reassure even those experienced in battle but who had never before faced a Jewish adversary.

"How are you?" Nachmani asked Marom. "Okay?"

"Okay, okay," Marom replied.

At the back entrance to this settlement of three synagogues, a yeshiva, or Jewish religious academy, built in the shape of a Star of David and well-kept homes, dozens of army buses pulled up just after 10 a.m. Nachmani's company began walking toward a crowd at the gates.

The soldiers moved along a concrete wall separating the settlement from the crossing into Khan Younis. On a hilltop ahead, young men and women, singing in high voices, locked elbows in a human chain. The soldiers reached the barrier, which then closed around them. For nearly 20 minutes, soldiers and settlers mingled in the small enclave formed by the human chain and the wall.

The soldiers retreated and soon were scuffling in the road with a number of young men, several of whom punctured tires of an arriving army jeep seconds before being chased away. "All the boys you see here yelling -- they will be soldiers or already are soldiers," said Marilyn Adler, 44, who arrived here from the West Bank settlement of Efrat last month with four of her six children.

Adler and her 8-year-old son, Itai, watched as disengagement opponents confronted a line of soldiers only slightly older in most cases. One of the opponents screamed, "Nazi! Nazi!" in a soldier's face. Another doused the windshield of a jeep with dishwashing soap before being slammed to the ground by an Israeli police official.

"I think this could work, I really do," Adler said of the opposition. "We're very determined. We'll see if the government gives up first."

The scuffling followed the same dynamic as recent anti-disengagement rallies that have mixed street theater and low-grade guerrilla tactics. Chanting and singing gave way to periods of rest when soldiers and settlers, in small groups, chatted quietly. A young opponent with a megaphone warned demonstrators to drink water and seek shade.

Hours later, as Company 2 rode back to the base in silence, Marom reflected on the afternoon. He no longer had to imagine what confronting his fellow settlers would entail. But he worried about what the days ahead would bring.

"It's going to be bad," he said. "Really, it's going to be bad."

Special correspondents Samuel Sockol and Ian Deitch contributed to this report.